1. Field of the Invention
The present invention relates to a seismic isolator which is interposed between a structure and its foundation to absorb and hence damp earthquake vibrations which are transmitted to the structure through the foundation and, more particularly, to a seismic isolator which absorbs and attenuates earthquake vibrations in a particular direction and is suitable for use with a bridge or similar structure.
2. Description of the Prior Art
Heretofore there has been known a seismic isolator of the type in which elastic laminated members are interposed between a structure and its foundation to support the vertical load of the structure so that external forces in the horizontal direction caused by earthquake are absorbed and attenuated by horizontal displacement of the elastic laminated members to buffer the earthquake vibrations.
The above-mentioned elastic laminated member is of a multi-layer structure comprising thin flat metal sheets and rubber sheets stacked alternately with each other and fixed together. The elastic laminated members are each fixedly secured at upper and lower ends to a building or similar structure and its foundation, respectively, and have the same rigidity, and consequently the same seismic isolating function, in any horizontal directions (Japanese Pat. Publication No. 34276/88).
It may sometimes be desirable, according to the kind of a structure, to attenuate earthquake vibrations only in a particular direction. For example, in the case of a bridge in which expansion joints are used, it is required to isolate vibrations only in the axial direction of the bridge (i.e. lengthwise direction thereof) but not to isolate vibrations in a direction perpendicular thereto. Consequently, when the above known seismic isolator is used with the bridge, stoppers must be provided in order to restrict the relative displacement of the bridge in a direction perpendicularly to the axis of the bridge.